Fish rely on forests for their very survival. That’s because, in a way, they eat them. Debris from forests finds their way into rivers, lakes, and streams. The bacteria in the water break down the leaves and bits of tree bark and dead animals. Then the zooplankton eat those bacteria, and the fish eat the zooplankton.

The “boreal zone” – that is, the sub-Arctic areas of places like Canada and Russia contain more than 60% of the planet surface’s fresh water. What will happen to the fish living in that water, wondered Laurentian University researcher Andrew J. Tanentzap and colleagues, in a world increasingly devoid of healthy forests?

Most food webs in those lakes and streams rely almost entirely on organic matter from forests, because they’re so nutrient-poor themselves. Without the scraps of plant and animal matter to feed the bacteria in the water, those food webs could collapse. Better understanding the complex relationships between aquatic communities and terrestrial ecosystems will allow us to better predict the consequences of deforestation, and might even give us some hints for how to mitigate that damage.

The researchers focused on a 2.75 square kilometer watershed in Ontario, Canada. The eight study sites they chose had varying levels of nearby forest cover. In particular, they looked at the yellow perch Perca flavescens. Carbon that originally came from forest debris has a different mass than carbon that originally came from algae. By taking tissue samples of the fish, the researchers could therefore parse out how much of that fish’s diet consisted of zooplankton that fed on algae versus on forest debris. They found that 34% of the fish biomass originated in forest debris, at minimum. That figure increased to 66% for fish from areas surrounded by better, more complete forest cover. While zooplankton that feed on algae are more nutritious, those that feed on forest debris make up a significant volume of the perches’ diet. Moreover, the fish from parts of the lake without sufficient forest cover were smaller, which makes them less likely to survive and to breed. The more nearby forest, the healthier, the bigger, and the more adaptive the fish were. Or, put another way, the less nearby forest, the scrawnier, less adaptive the fish were.

Since some 7% of the protein in human diets comes from fish – and its a higher proportion in poorer communities – that’s a bit worrisome. “Our results now suggest that anthropogenic disturbances, such as fire and logging, which are increasingly removing large amounts of plant biomass from boreal forests, should reduce export of terrestrial organic matter and subsequently the biomass of aquatic food webs,” writes Tanentzap. Species and ecosystems do not exist in isolation, but are inextricably linked, even if those links are not immediately obvious. The downstream results of major landscape alterations – like deforestation – will include consequences that we couldn’t even begin to predict. – Jason G. Goldman | 20 June 2014